An ultrasound system has become an important and popular diagnostic tool since it has a wide range of applications. Specifically, due to its non-invasive and non-destructive nature, the ultrasound system has been extensively used in the medical profession. Modern high-performance ultrasound systems and techniques are commonly used to produce two or three-dimensional images of internal features of an object (e.g., human organs).
Generally, the ultrasound image is displayed in a Brightness-mode (B-mode) by using reflectivity caused by an acoustic impedance difference between the tissues of the target object. However, if the reflectivity of the target object is hardly different from those of the neighboring tissues such as tumor, cancer or the like, then it is not easy to recognize the target object in the B-mode image.
To cope with the problem of recognizing the tumor, cancer and the like in the B-mode, an ultrasound elasticity imaging has been developed to visualize the mechanical characteristics of the tissues based on differences responsive to pre-compression and post-compression. Such imaging proved very helpful for diagnosing lesions such as tumor and cancer, which otherwise are hardly recognized in the B-mode image. The ultrasound elasticity imaging may utilize the scientific property that the elasticity of the tissues is related to a pathological phenomenon. For example, the tumor or cancer is relatively stiffer than the surrounding normal tissues. Thus, when stress is uniformly applied, a strain of the tumor or cancer may be typically smaller than those of the surrounding tissues. Strain is deformation of a target object due to stress applied per area and Young's modulus may be defined as a ratio of stress over strain. The strain is a differential value of a displacement. The displacement may indicate how much tissues in the target object are moved between pre-compression and post-compression.
The ultrasound system may set a window on each of the pre-compression frame data and post-compression frame data, and move the window in an axial direction for a correlation operation therebetween, thereby obtaining displacements. In such a case, when a position gap between the windows set on the pre-compression frame data and the post-compression frame data is beyond a range of a phase, a decorrelation error may occur.
Conventionally, the ultrasound system is configured to determine an initial displacement from a current position of the window (i.e., displacement that has already been computed in an axial direction at a current position of the window) and move the window by the initial displacement to compute a displacement in order to remove the decorrelation error. In such a case, however, if an error in the initial displacement occurs, then an error is maintained in an axial direction. Thus, a dropout, i.e., a horizontal line in a strain image, may be generated.